Active array antennas are coming into increased use because of their adaptability, low inertia and multi-beam capability. In an active array antenna, each antenna element is associated with a "transmit-receive" (TR) module, which amplifies the signal received by the antenna element to provide a good noise figure and to compensate for losses which occur in the receive beamformer. The TR module may also include a transmit amplifier, which amplifies transmit signals arriving at the TR module from the transmit beamformer, so that each antenna element radiates an amplified signal. The amplified signals radiated by the antenna elements "combine in space" to produce the net transmit power. An air traffic control radar system using active array antennas is described, for example, in U.S. Pat. No. 5,103,233, issued Apr. 7, 1992 in the name of Gallagher et al, incorporated herein by reference.
FIG. 1 is a perspective or isometric view of a shelter as described in the abovementioned Gallagher et al. patent, adapted for supporting phased-array antennas. In FIG. 1, structure 10 is in the form of a truncated quadrilateral pyramid including faces or sides 12 and 14. Structure 10 sits atop a base or foundation 16. Each face 12, 14 of structure 10 bears a planar array antenna 18. Array antenna 18a is associated with face 12, array antenna 18b is associated with face 14, and two other array antennas are associated with the two hidden faces. Those skilled in the art of array antennas know that array antennas such as antenna 18 may be two-dimensional arrays of hundreds or thousands of antenna elements, and may be of any shape, including the illustrated trapezoidal shape, or rectangular, circular, elliptical, or may even be annular or of some other shape. As described in the abovementioned Gallagher et al. patent, the TR modules and antenna elements of each array may be fed by a corporate feed. Corporate feeds are well known in the art, being described, for example, in U.S. Pat. No. 5,017,927, issued May 21, 1991 in the name of Agrawal et al., which is incorporated herein by reference.
In FIG. 1, a direction broadside (normal to the surface of) to the surface of array 18b is illustrated as dash line 24, which makes an angle .theta..sub.T with the horizontal x axis.
When structure 10 of FIG. 1 houses an air traffic control radar system as described in the abovementioned Gallagher et al. patent, economic considerations dictate that it may often be the only all-weather aircraft control system available, and must be very reliable. In the context of a shipboard fleet self-defense radar such as the AEGIS system currently in use, the importance of reliability cannot be overstated. Thus, a radar arrangement and its housing may be required to be very reliable and easily maintained.